From the viewpoint of energy problems and environmental problems, it is required to efficiently reduce CO2 by light energy like plants. Plants use a system, called Z-scheme, which excites light energy in two stages. In a photochemical reaction using this system, water (H2O) is oxidized to obtain electrons, and carbon dioxide (CO2) is reduced to synthesize cellulose or sugars.
Currently, artificial photosynthetic systems which imitate the Z-scheme of such plants have been developed. In the artificial photosynthetic system, a potential necessary for reduction of CO2 is obtained from visible light by using a photocatalyst. However, since energy efficiency of the photocatalyst is low, an artificial photochemical reaction which does not use a sacrificial reagent has a problem that reaction efficiency is very low. Therefore, a CO2 reduction technology with high reaction efficiency is required.